The design of transit vehicle doors is crucial for the overall safety of the travelling public and they must function in a rather hostile environment. Such hostile environment at least includes heavy usage, temperature extremes, both vibration and acceleration loads, as well as spurious electrical signals which may be caused by lightning, by an interrupted contact with a third rail, and/or strong radio signals.
In many applications, an automatic actuation of the doors is required. An actuator for such a door must open and close the door without failure for a very large number of cycles. Also, a positive locking mechanism is generally needed. The locking mechanism should prevent accidental opening of the doors by any application of opening forces to the doors when the doors are closed.
Some prior art systems rely on the actuator to move the door to a closed position and then the actuator is locked to lock the door. This approach has the disadvantage that the locking loads are carried by arms mounted on the actuator which position the door in the closed position. This approach does not provide strong, solid locking.
A prior art unit made by Bode.sup.a has an actuator which has a vertical axis. A piston in the actuator which is driven upward by fluid pressure moves a screw which engages a nut. The nut supports and positions arms which, in turn, support and position the door. When the actuator begins a door-closing stroke, the door is rotated to the closed position. When it reaches the closed position, it can rotate no further. Continued displacement of the piston then moves the screw upward, moving the door with it. This upward motion of the door engages locks on the edges of the door. Then, if a force is applied to the door in the door-opening direction, the locks carry the load, not the arms. One drawback of this approach is that during the final upward movement of the door the actuator is exerting strong door closing forces. These strong door closing forces cause friction, which increases the force needed to lift the door.